The present invention relates generally to superplastic forming, and particularly to control the rate at which the part being formed is strained by monitoring locations of the part that form most rapidly when forming pressure is applied.
Superplastic forming involves the forcing of a blank of sheet metal into a female die cavity over a male die form in the cavity at a certain temperature value and strain rate. In the superplastic forming of aluminum, for example, the formability thereof decreases and the tendency to cavitate increases as the strain rate of the forming operation deviates from an optimum strain rate. (Cavitation is the formation of internal voids in the material of the part being formed.) The suppression of cavitation with increases in back pressure, such as taught in U.S. Pat. Nos. 4,354,369 and 4,516,419 to Hamilton and Agrawal respectively, becomes more difficult as the deviation from the optimum strain rate increases. In order to optimize the superplastic forming process for aluminum, including the elimination of internal voids, a schedule of pressure versus time that will result in an optimum rate of strain must be employed.
A pressurization schedule which will result in successful superplastic forming is often developed through trial and error. A method of analytically predicting the pressurization schedule needed for optimum superplastic formability and then control of the forming process to maintain such a schedule is described in U.S. Pat. No. 4,181,000 to Hamilton et al. Further details of the principles described in the Hamilton et al Patent are disclosed in U.S. Pat. Nos. 4,233,829 and 4,233,831 to, again, Hamilton et al.
The disclosures of the above patents are incorporated herein by reference.
The configuration of most components made by superplastic forming are quite complex. (The drawings of the above patents do not depict such complexity.) The strain rate will thus vary from location to location of the blank, from which the component is made, during the forming process for a given pressurization schedule. What is therefore needed is a method to determine critical locations in the part at various times during the schedule, then control the pressurization schedule in a manner that will result in an optimum strain rate at the fastest deforming locations and maintain that rate and schedule.
Sophisticated models of the Process of superplastic forming, based upon finite element analysis, have been developed, including a model developed by Dr. M. P. Sklad, one of the inventors of the present application. Such models are used to predict one or more locations in a component that are deforming most rapidly at a given instant of time during the forming process. The schedule of pressurization needed to bring the strain rate at these critical locations to the optimum can then be calculated.
The stress in the blank that results from the pressure of the forming fluid applied to the blank is strongly affected by the thickness of the blank at a given point in time of the forming schedule. It is the stress in the material of the blank that produces strain rate. Slight errors in the predicted pressures or in control of the pressures can cause in-plane stresses and resulting strain rates to be much higher than expected. If an analytically determined pressure-time schedule is strictly adhered to, such small errors in predicted pressures can cause the forming process to go out of control.
What is therefore needed in the art of superplastic forming, and which forms an objective of the present invention, is the ability to sense in real-time the strain rate occurring at the critical locations provided by an analytical model such that the actual strain rates can then be calculated and employed in a closed-loop feedback fashion to control the rate in which pressurization is applied in the forming process.